1. Field of the Invention
The present invention generally relates to the art of power supplies and more particularly, to a regulated power supply with power factor correction (PFC) control.
2. Description of the Related Art
Regulated power supplies are used for power conversion in many applications, including computers, lighting ballasts, and telecommunications devices. Products consuming 70 watts or more generally require regulated power supplies with power factor correction, to reduce power loss and comply with environmental regulations. In these and other products where significant load variations are frequent, regulated power supplies capable of reacting rapidly to sudden load changes are especially desirable.
Without power factor correction, an AC/DC power conversion system will draw current through the rectifier in sharp bursts, shown in FIG. 1B. These high peak currents cause significant power losses due to heat dissipation. Furthermore, they can put heavy stress on the power distribution system and the transmission lines.
A power factor correction circuit can almost eliminate these current ripples by regulating the input current with a feedback control loop. The power factor correction circuit synchronizes the rectifier input current with the rectifier voltage output (FIG. 1A) and the power supply voltage output. The power supply can still provide the same constant voltage output power with a continuous and low-peak input current. FIG. 1C demonstrates a power supply's input current waveform with power factor correction. The lower peak currents enable the power supply to convert energy very efficiently, while minimizing the stress on the power distribution system and the transmission lines.
To generate a low distortion input current during steady-state operation, a regulated power supply needs a power factor correction section with a very low bandwidth error amplifier. The low bandwidth error amplifier filters out non-DC components from the power supply output voltage, so that they are not introduced back into the feedback control loop. Because the input to the power supply is an AC signal, the output, despite being a DC signal, will inevitably still contain an AC component. Within limits, this is acceptable on the output, but for the power output to remain stable, this component must be removed as much as possible from the feedback signal. Since the AC component is a low frequency signal (60-120 Hz), a very low bandwidth error amplifier is required in the power factor correction circuit to do this.
Regulated power supplies must also respond quickly to rapid transients. These can occur whenever the output load changes, the power supply turns on or off, and when the supply input is affected by glitches or surges. If the power supply does not react fast enough in these situations, the output voltage will also change, possibly beyond the predetermined operating range of the power supply. This can result in an untimely shut down and possible circuit damage.
Unfortunately, achieving low signal distortion and fast response have traditionally been conflicting goals of regulated power supply design. A power factor correction circuit with an error amplifier configured to provide low distortion will react very slowly to load changes. By the time the output of the power supply is corrected, either high or low voltage protection alarms will be reached, and the power supply will shut down. This is because a typical low bandwidth error amplifier filters out higher frequency signals from the feedback loop, meaning that it is a ‘slow’ component. While this is necessary for eliminating current distortion, it reduces the responsiveness of the power factor correction circuit.
Prior-art regulated power supply systems have attempted to address these conflicting goals by making compromises through the careful selection of components in the power factor correction circuit. However, such compromises make it impossible to simultaneously achieve optimum performance in current distortion and transient response. For the reasons described above, designing a power factor correction circuit that simultaneously provides low signal distortion and rapid transient response is fundamentally difficult. Low current distortion requires a ‘slow’ error amplifier, but rapid transient response requires a ‘fast’ error amplifier.
The challenge is to design an error amplifier for a power factor correction circuit that filters out AC component ripple during steady-state operation, while quickly and smoothly responding to sudden changes in the output load and the supply voltage. The ideal would be a flexible circuit that could detect rapid transients, and temporarily increase its control bandwidth in response without increasing gain.
One method of addressing this problem is disclosed in U.S. Pat. No. 5,619,405. Kammiller et al. discloses a power factor correction circuit with variable bandwidth control. The invention comprises of a variable resistance connected to an input of a low bandwidth amplifier, and control circuitry for switching the variable resistance in response to output conditions. When the control circuit senses a change in the output load, the resistance connected to the input of the low bandwidth amplifier can be decreased temporarily by a switching mechanism. This allows the feedback-control circuit to temporarily operate at a higher bandwidth for improved transient response.
One drawback of the Kammiller invention is that it fails to decouple steady-state operation from transient-mode operation. During steady-state operation, feedback signals pass through a low bandwidth amplifier. To allow control signals to propagate faster in response to rapid transients, Kammiller introduces a novel bandwidth control switching mechanism. This design can increase the overall speed of the feedback loop, but only to a limited degree. The output of the bandwidth control mechanism is connected in series to the input of the low bandwidth amplifier. Transient-mode feedback control signals are still severely bandwidth-limited by what is effectively a low-pass filter. The transient response of the circuit is still subject to limitations imposed by the requirements of steady-state operation.
Another drawback of the Kammiller invention is that it is prone to instability. The resistance switching mechanism claimed by Kammiller does increase the control bandwidth of the feedback loop whenever the output voltage exceeds steady-state boundaries. However, by reducing the resistance attached to the input of the low bandwidth amplifier at higher circuit frequencies, the resistance switching mechanism also increases the overall gain of the feedback circuit. FIG. 2A illustrates the gain characteristic of the Kammiller invention. It is well known to those skilled in the art that simultaneously increasing power gain and bandwidth tends to cause a feedback control system to become unstable. Thus, oscillations in the output voltage may be observed during on/off and load change transients. Furthermore, the said resistor switching mechanism will result in very abrupt and sudden transitions, further putting stress on the circuit and endangering stability. FIG. 3A illustrates the transient response of the Kammiller design.
Another drawback of the Kammiller invention is that the transient response is slow. The low bandwidth amplifier introduces a phase delay into the power factor correction control signal. The low bandwidth amplifier consists of an amplifier connected to a capacitance. Despite the resistance switching mechanism, transient feedback signals must still pass through this component and suffer a phase delay.
Another drawback of the Kammiller invention is high production cost. As explained above, the transient response of the Kammiller design is susceptible to instability. The maximum output voltage during transient-mode could be substantially higher than the steady-state output voltage. To cope with this, it would be necessary to use a bank capacitance with a high voltage rating on the output side of the power supply. If the power supply were built to output 385V-400V DC, it would be necessary to use a bank capacitance rated at 450V. The increased cost of this is very high relative to the overall cost of the circuit.
Finally, the Kammiller invention does not disclose how to build said resistor switching mechanism that its claims rely upon. There are no diagrams that show how to construct this component. Neither there is any detailed explanation or description in any of the preferred embodiments. Without knowing how to build the invention, it is difficult to assess. A simple way of designing the said switching mechanism is not known to the art. Any practical implementation of the variable resistance mechanism will be cumbersome and expensive relative to the overall cost of the circuit. Furthermore, the switching mechanism may introduce other complications that would need to be addressed. In the absence of a full disclosure of the method of constructing the said device, it is necessary to conclude that the Kammiller invention has not conclusively solved the problems described above.
It will be apparent to those skilled in the art that both of the preferred embodiments of the Kammiller invention exhibit the above stated shortcomings. Thus, a need still remains for a power factor correction circuit that provides low current distortion with a fast and stable transient response.